Hidden organic optoelectronic devices with a light scattering layer

ABSTRACT

An optoelectronic device ( 100 ) comprising at least one optoelectronic active region ( 101 ) comprising at least a rear electrode ( 102 ) and a front electrode ( 103 ) between which an organic optoelectronic material ( 104 ) is sandwiched, said rear electrode ( 102 ) being reflective, and a cover layer ( 105 ) arranged in front of said front electrode ( 103 ). The cover layer ( 105 ) comprises a material with light-scattering particles ( 110 ) of a first material dispersed in a transparent matrix ( 111 ) of at an least partly hydrolyzed silica sol. Due to the highly scattering propertied of the cover layer, the device is essentially concealed behind the cover layer when not in its operative state.

FIELD OF THE INVENTION

The present invention relates to an optoelectronic device comprising atleast one optoelectronic active region that comprises at least a rearelectrode and a front electrode between which an organic optoelectronicmaterial is sandwiched, said rear electrode being reflective, and acover layer arranged in front of said front electrode.

BACKGROUND OF THE INVENTION

OLED (organic light-emitting diode) and OPV (organic photovoltaics)technology is emerging as an alternative to different types ofillumination/recharging purposes. Collectively, OLEDs and OPVs arereferenced to as organic optoelectronic devices. In general, an organicoptoelectronic device comprises two electrodes between which an organicoptoelectronic material is sandwiched.

In an OLED, the optoelectronic material is an electroluminescentmaterial. When a current is made to flow between the electrodes, theorganic electroluminescent material emits light.

In an OPV device, the optoelectronic material is an organic photovoltaicmaterial, which collects photons and transforms them into negative andpositive charges so as to produce a voltage between the electrodes.

Due to the flexible nature of the organic optoelectronic devices, theymay advantageously be used in flexible applications, i.e. applicationswhere the device may be bent during normal operation, or on curvedsurfaces, for example providing a curved display device or illuminationsystem in the case of an OLED.

In this context, a drawback of at least the current technology is thatone of the anode and cathode electrodes that sandwich the optoelectronicmaterial is highly reflective in order to obtain high light utilization.Hence, the devices have a mirror like appearance, which is not desiredin some applications. For example, the appearance of the OLED in theOFF-state is important, and different solutions have been proposed inorder to improve it.

U.S. Pat. No. 6,501,218 to Duggal et al. describes a device structurefor outdoor signs utilizing OLED technology. Here, an OLED which ispatterned into a sign, such as a character or a number, is combined witha highly scattering, non-absorbing coating over the light-emitting OLEDregions and a highly absorbing coating over the non-emitting regions.The result is a sign that can be viewed by virtue of the OLED lightunder low ambient light level conditions thanks to the combination ofthe highly scattering material forming the sign (character, number) andthe highly absorbing coating forming the outline of the sign.

U.S. Pat. No. 6,501,218 discloses the use of a scattering enamel coatingon top of the OLED. However, the enamels need to be sprayed onto aplastic film or glass slide, which is then transferred to the OLEDdevice. There is a demand for a coating that can be directly applied tothe OLED surfaces without the need for intermediate coating steps.

The enamel coating of U.S. Pat. No. 6,501,218 has the furtherdisadvantage that it will crack or flake off when the substrate ontowhich it has been sprayed is stressed or bent.

Also, there is a high interest in obtaining large surfaces containingluminous patterns for decorative and informative purposes, and it wouldbe desirable in many cases if the patterns were visible only when theOLED surface emits light.

Furthermore, there is an interest in providing OPV devices which can bemade invisible to a user, for example so as not to disturb the visualappearance of the device which is provided with a voltage from the OPV.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate at least some ofthe problems of the prior art at least in part and to provide animproved organic optoelectronic device that is substantially concealedfrom a viewer unless it is in its operative state.

Thus, in a first aspect, the present invention provides anoptoelectronic device comprising at least one optoelectronic activeregion, comprising at least a rear electrode and a front electrodebetween which an organic optoelectronic material is sandwiched, saidrear electrode being reflective, and a cover layer arranged in front ofsaid front electrode. Said cover layer comprises a material comprisinglight-scattering particles of a first material dispersed in atransparent matrix of at an least partly hydrolyzed silica sol.

The cover layer has a highly scattering characteristic due to thescattering particles. Hence, it has a high hiding power such thatstructures arranged beyond the cover layer are not visible. However,light can pass through the layer.

An at least partly hydrolyzed silica sol has a strong resistance tocracking under stress and can hence advantageously be used when theoptoelectronic device is likely to be subjected to stresses.

An at least partly hydrolyzed sol can be conveniently obtained throughdrying of a prehydrolyzed silica sol, which can be done at roomtemperature. This cover layer material can be obtained without solventsand/or high temperatures that would otherwise have a negative impact onthe OLED function. Furthermore, it is essentially non-absorbing, and theinclusion of scattering particles makes the cover layer highlyscattering.

In embodiments of the present invention, the optoelectronic material maybe an electroluminescent material.

If the optoelectronic material is an electroluminescent material, theoptoelectronic device of the present invention is an OLED (organiclight-emitting diode) device. The OLED emits light through the coverlayer into the surroundings.

The light emitted by the OLED is received by the cover layer, and aportion (T) of this light is transmitted through the cover layer.Another portion of the light (1−T) is reflected back towards the OLED.This portion of this light (R(1−T)), where R is the reflectivity of thereflective electrode, is once more received by the cover layer afterreflection in the reflective electrode of the OLED. A portion of this,secondary, light T(1−T)R is transmitted through the cover layer, whereasanother portion T(1−T)²R is reflected back towards the OLED. Thiscontinues until there is no light left to be transmitted through thecover layer. As a result, the portion of the light emitted by the OLEDand transmitted through the cover layer is significantly higher thanwould be expected on the basis of the transmissivity of the cover layer.Thus, in operation, the light emitted by the OLED is clearly visiblethrough the cover layer. In the non-operative state, however, the OLEDstructure will be essentially invisible through the cover layer.

In other embodiments of the present invention, the organicoptoelectronic material may be an organic photovoltaic material.

If the optoelectronic material is an organic photovoltaic material, thedevice of the present invention is an OPV (organic photo voltaicdevice), capable of transforming light into an electrical voltage.

Arranging an OPV behind a cover layer in accordance to the presentinvention renders the structures of the OPV invisible to the viewer, sothat they can be hidden in different devices in which such an OPV isrequired. The OPV works well with diffused light, so the scatteringcharacteristics of the cover layer do not hamper the function of theOPV.

It is noted that a device according to the present invention maycomprise both an organic electroluminescent material and an organicphotovoltaic material, for example one domain of the device acting as anorganically based light-emitting device and another domain of the deviceacting as an organically based solar cell.

In embodiments of the present invention, said cover layer issuperimposed on said at least one optoelectronic active region andcovers at least the entire surface of said at least one optoelectronicactive area.

Covering the entire optoelectronic surface with the cover layer hidesthe optoelectronic device, i.e. makes it essentially invisible to aviewer. In an OLED device, it may remain concealed until the device isin its operative state.

In embodiments of the present invention, the device may comprise atleast a first and a second optoelectronic active area, which areas arearranged side by side and are mutually spaced apart so as to form aninterstitial region there between, wherein said cover layer issuperimposed on said first and second optoelectronic active regions andcovers at least the combined surface of said first and secondoptoelectronic active regions and said interstitial region.

Covering of two or more OLEDs as well as the interstices between theOLEDs with one and the same cover layer provides a light pattern thatcan be displayed on the surface of the cover layer even though, asdiscussed above, the light-emitting device is concealed until in itsoperative state. In the case of a photovoltaic device, the surface ofthe cover layer will not reveal the presence of a plurality of devicesarranged behind it.

In embodiments of the present invention, said transparent matrix is asilica sol-gel.

A sol-gel may be obtained by further drying of a partly hydrolyzedsilica sol. This can be done at room temperature, or at least attemperatures that do not damage the optoelectronic components, and alsowithout the use of chemical compounds such as solvents, which aredetrimental to the optoelectronic components. A silica sol-gel isfurther a glass-like material which has a good resistance to mechanicalinfluences, such as scratching.

In embodiments of the present invention, said cover layer may have areflectivity in the range of from 50 to 95%.

Preferably, the reflectivity of the cover layer is within said range inorder to maintain the compromise between the ability to hide thestructure of the optoelectronic device(s) and the ability to emitsufficient light. In the case of an OLED device, the OLED is hidden inthe non-operating state, while the cover layer allows light emitted bythe OLED(s) to pass through.

In embodiments of the present invention, the refractive index of saidparticles of said first material is higher than the refractive index ofthe transparent matrix.

A good scattering effect is obtained by dispersing particles of highrefractive index in a material of low refractive index.

In embodiments of the present invention, said particles of said firstmaterial account for about 10 to about 80% by weight of said cover layermaterial, preferably 15 to 70% by weight.

The light-scattering particles are contained in the cover layer at theabove concentration in order to give the cover layer a good scatteringeffect.

In a second aspect, the present invention relates to an arrangementcomprising a device of the first aspect of the invention arranged in aframe, at least partly surrounding the lateral edges of said device,wherein said cover layer covers said at least one device and at leastpart of said frame.

Covering of both the optoelectronic device(s) and the frame around itwith the same cover material renders it possible for the optoelectronicdevice to be efficectively hidden, since the transition from the frameto the OLED will not be easily detected from the outside by a merevisual inspection in the non-operating state (of an OLED).

In a third aspect, the present invention relates to a method for themanufacture of an optoelectronic device according to the invention,comprising the steps of providing an optoelectronic device; providing anoptionally prehydrolyzed silica sol with particles of said firstmaterial dispersed therein; arranging a layer of said silica sol infront of said front electrode of said optoelectronic device; and dryingsaid layer.

It is further noted that the invention relates to all possiblecombinations of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing acurrently preferred embodiment of the invention.

FIG. 1 schematically illustrates a light-emitting device of the presentinvention in a cross-sectional view.

FIG. 2 a schematically illustrates an OLED-based arrangement of thepresent invention in OFF-state in plan view.

FIG. 2 b schematically illustrates the arrangement of FIG. 2 a inON-state in plan view.

DETAILED DESCRIPTION

The present invention relates to an organic optoelectronic devicecomprising a pair of electrodes, a rear and a front one between which anorganic optoelectronic material is sandwiched, the rear electrode beingreflective, while a cover layer is arranged in front of the frontelectrode.

A light-emitting device 100, i.e. an organic optoelectronic devicewherein the organic optoelectronic material is an electroluminescentmaterial according to the present invention, is schematicallyillustrated in FIG. 1 and comprises two OLED active regions 101 and101′. Each OLED active region 101, 101′ comprises an organicelectroluminescent material 104 arranged, i.e. sandwiched, between arear electrode 102 and a front electrode 103. An OLED active region 101is defined as a region in which the electroluminescent material 104 issandwiched between the two electrodes 102, 103. Regions between adjacentOLED active regions 101, 101′ are denoted interstitial regions 106hereinafter.

As used herein, the term “arranged in front of” in the context of thecover layer being arranged in front of the front electrode, means thatthe cover layer is arranged between the front electrode of the deviceand the external surroundings of the device. For an OLED device thismeans that the cover layer receives light emitted by the OLED and passesthis on to the surroundings. For an OPV device, this means that ambientlight passes through the cover layer before it passes through the frontelectrode and reaches the photovoltaic layer.

In the embodiment of FIG. 1, the front electrode 103 is transparent, sothat it represents the light-emitting (front) side of the OLED device,whereas the rear electrode 102 is reflective.

Materials suitable for the electrodes and the organic electroluminescentor organic photovoltaic material will be known to those skilled in theart and will not be discussed in detail here. Typically, though, thetransmissive front electrode may made of a transparent electricallyconducting material such as ITO (indium-tin oxide), and the reflectiverear electrode may be made of a reflective electrically conductingmaterial such as a metal or a metal-coated material.

The organic optoelectronic material may be a polymeric material or amaterial with small organic molecules, as is commonly known in the art.

As is known in the art, an optoelectronic device may furtherconventionally comprise additional layers, such as barrier layers, metalshunts for uniform current distribution, buffer layers, and substrates.For the sake of simplicity, however, the description of such layers isomitted as their location and use are well known to those skilled in theart. An optoelectronic device, such as the light-emitting device 100further typically comprises driving electronics (not shown),conventional in the art.

A cover layer 105 is arranged on top of the OLED active regions 101,101′, in front of the front electrode 103, and also covers theinterstitial region 106 located between these active regions.

The cover layer 105 is arranged on a substrate 107 located between thefront electrode and the cover layer 105. The substrate may, for example,be of glass or plastics and may, for example, comprise buffer layersprotecting the active layers from water and/or oxygen.

The cover layer comprises an essentially non-absorbing matrix 111 of anat least partly hydrolyzed silica sol-gel in which scattering particles110 are dispersed.

Typically, the matrix 111 is a silica sol-gel, which has the advantagesof being a transparent, hard, scratch-resistant and glass-like material.

The scattering particles 110 are typically of a material having arefractive index higher than that of the surrounding matrix 111. Forexample, the refractive index of the scattering particles is preferablyat least 2.0. The surrounding matrix typically has a refractive index ofabout 1.3 to 1.6.

The scattering particles 110 are typically made of a material selectedfrom the group consisting of TiO₂ anastase, TiO₂ rutile, ZrO₂, Ta₂O₅,ZnS, ZnSe, or mixtures of two or more thereof.

These materials are good examples of materials suitable for essentiallynon-absorbing, scattering particles to be dispersed in said matrix.

The scattering particles typically account for about 10 to about 80% byweight of said cover layer material, preferably 15 to 70% by weight.

The light-scattering particles are contained in the cover layer in aconcentration within the range given above so as to provide a goodscattering effect in the cover layer.

The particle size of the scattering particles 110 can be selected tomatch the color of light emitted by the OLED so as to obtain a maximumscattering effect. The mean particle size should be close to thewavelength of the emitted light for this. Hence, the mean particle sizeof the scattering particles ranges of from 100 to 1000 nm, preferably200 to 800 nm (i.e. the wavelength range from UV to visible light.)

The concentration of scattering particles 110 in the matrix and thethickness of the cover layer 105 are typically chosen so as to obtain acoating which conceals the OLED structures when the OLED active regionsare in a non-operating (OFF) state, but which allows light emitted bythe OLED active regions to shine through the cover layer 105.

Typically, a coating layer having a reflectivity of above 50% per passis desired, preferably above 75%, such as above 85%, whereby a goodhiding power is obtained.

The thickness of the cover layer is typically from 1 μm to 50 μm inorder to provide a good hiding property and desired transmissivity.

In an ideal case, assuming a non-absorbing cover layer, the totaltransmission of light emitted by the OLED through the cover layer can becalculated as

$T_{tot} = {\sum\limits_{n = 0}^{\infty}{T\left( {\left( {1 - T} \right)R} \right)}^{n}}$

where T is the transmission per pass through the cover layer (1−thereflectivity) and R is the reflectivity of the reflective electrode ofthe OLED active region.

For a value of T of 20% (80% reflection) and R of 80%, which arerepresentative values of devices of the present invention, T_(tot)equals 0.6.

Hence, the light emitted by the OLED active regions will be clearlyvisible through the cover layer, while the OLED structures will besubstantially invisible through the cover layer in the OFF-state. Thecover layer 105 may be obtained, for example, as follows.

A silica precursor sol-gel is obtained by prehydrolyzing an alkoxysilanesolution in water, for example with an acid acting as a catalyst.

A suspension of silica particles is added to the prehydrolyzed sol. Thenthe scattering particles 110 are added to the mixture.

The resulting mixture can than be homogenized by means known to thoseskilled in the art, such as a roller bench. The result is a stablesuspension. If kept in a freezer, the suspension has a shelf life of atleast two months.

The suspension may then be coated onto the OLED surface by means of anyconventionally used coating method, such as e.g. spin coating, spraycoating or doctor blade coating.

The coated layer is allowed to dry in room temperature, no further heattreatment being needed to obtain a hard protective cover layer with thedesired compromise between transmission and hiding properties.

The OLEDs used may be a uniform tile or may have a patterned picture orother atmosphere-creating design that is visible before coating.

It will be apparent to those skilled in the art that the embodimentsdescribed above may also be applied in an organic photovoltaic device(OPV) if the organic electroluminescent material is replaced by anorganic photovoltaic material.

Typically, the OPV (also known as organic solar cell) is used fordriving an electronic device of some kind (e.g. an OLED) by convertinglight, such as sunlight or indoor light, into electrical energy. An OPVof the present invention, which is concealed from a viewer, isadvantageous in many applications, for example where it is desired thatthe OPV should not interfere with the visual appearance of the device towhich the OPV provides a voltage. Since the OPVs are not visibleanymore, the applicability of this method from a design point of view isenhanced. Examples of such applications include, but are not limited to,a solar-cell-driven watch, PDA, mobile phone, etc., where the OPV actsas the solar cell.

For the function of an OPV, the light utilized may very well bescattered, as this does not affect the light conversion efficacy of thedevice much. Hence, the OPV may advantageously be located behind a coverlayer, as in the present invention.

However, if the optoelectronic device of the present invention is anOPV, the transmissivity of the cover layer is preferably not asrestricted as described above in the OLED embodiment. Instead, the coverlayer is typically selected to have a transmissivity of at least 20% perpass (a reflection of below 80% per pass), such as at least 50% perpass. However, the cover layer is typically selected so as to have goodhiding characteristics, i.e. to be highly scattering.

The device of the present invention may be embedded in a surface, atleast partly surrounded by a frame of the surface material, where boththe device and the frame material are coated by the same cover layermaterial. Hence, the location of he device will be concealed from aviewer (in the case of an OLED device, at least until the device is inits ON-state and emits light). This may be used in a multitude ofapplications, as required, for quickly presenting information, a warningsign, or an artistic pattern on a wall.

Such an arrangement, comprising a light-emitting device 100 of thepresent invention embedded in an ordinary wall 200, is illustrated inFIGS. 2 a and 2 b. In FIG. 2 a the light-emitting device is in theOFF-state, and the dashed lines only indicate the location of thelight-emitting device 100. A opening is cut in the wall 200 to form aframe within which the light-emitting device 100 is arranged. The coverlayer 105 is used not only to coat the light-emitting device 100 butalso to coat the wall 200.

In FIG. 2 b, the text “FIRE EXIT” and an arrow pointing in the desireddirection is lit on the wall when the light-emitting device 100 is ON.This obviously represents only one possible use of an arrangement of alight-emitting device arranged in a frame of surrounding material.

The cover layer material based on a silica sol-gel as used in thepresent invention does not only adhere to OLED surfaces, but also toother surfaces such as, but not limited to, glass, metal, ceramic,plastic, or wooden surfaces. Hence, the light-emitting device of thepresent invention can be arranged in a frame of virtually any material.As OLED light-emitting devices can be made in flexible/bendableembodiments, the light-emitting devices of the present invention may bearranged in curved surfaces, such as a pillar, or the like.

Those skilled in the art will realize that such an arrangement of anoptoelectronic device partly surrounded by frame, where both the deviceand the frame material are coated by the same cover layer material, isalso applicable to an OPV device.

In the embodiments described above, the cover layer covers more than theactive regions of the optoelectronic devices. In other embodiments ofthe invention (not shown), however, the cover layer only, or essentiallyonly covers the active region(s), such that the shape or patters of theactive region(s) is clearly visible even though the actual layerstructure of the optoelectronic device is concealed by the cover layer.For example, the cover layer may be made clearly visible through theintroduction of a dye or pigment.

Those skilled in the art will realize that the present invention is byno means limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims.

For example, the cover layer material may contain a pigment or dye togive the cover layer a desired color.

1. (canceled)
 2. A device according to claim 16, wherein saidoptoelectronic material is an electroluminescent material.
 3. A deviceaccording to claim 16, wherein said organic optoelectronic material isan organic photovoltaic material.
 4. A device according to claim 16,wherein said cover layer is superimposed on said at least oneoptoelectronic active region and covers at least the entire surface ofsaid at least one optoelectronic active area.
 5. (canceled)
 6. A deviceaccording to claim 16, wherein said transparent matrix is a silicasol-gel.
 7. A device according to claim 16, wherein said cover layer hasa reflectivity in a range of 50 to 95%.
 8. A device according to claim16, wherein the refractive index of said particles of said firstmaterial is higher than the refractive index of the transparent matrix.9. A device according to claim 8, wherein the refractive index of saidparticles of said first material is at least 2.0.
 10. A device accordingto claim 9, wherein said first material is selected from the groupconsisting of TiO₂ anastase, TiO₂ rutile, ZrO₂, Ta₂O₅, ZnSe, ZnS, andcombinations of two or more thereof.
 11. A device according to claim 16,wherein said particles account for about 10 to about 80% by weight ofsaid cover layer material.
 12. A device according to claim 16, whereinthe mean particle size of said particles of the first material lies in arange of 100 to 1000 nm.
 13. A device according to claim 16, whereinsaid transparent matrix comprises a color dye. 14-15. (canceled)
 16. Anoptoelectronic device, comprising a first and a second optoelectronicactive region arranged side by side and mutually spaced apart so as toform a interstitial region therebetween, said first and secondoptoelectronic active region comprising a reflective rear electrode, afront electrode, and an organic optoelectronic material disposedtherebetween, a cover layer superimposed on said first and secondoptoelectronic active regions facing said front electrode and coveringat least the combined surface of said first and second optoelectronicactive regions and said interstitial region, said cover layer comprisinga plurality of light-scattering particles of a first material dispersedin a transparent matrix comprising at least partly hydrolyzed silicasol.